Cyclone separators, which are sometimes referred to merely as cyclones, are devices that utilize centrifugal forces and low pressure caused by spinning motion to separate materials of differing density, size and shape. FIG. 1 illustrates the operating principles in a typical cyclone separator (designated by reference numeral 10 in FIG. 1). The following is a description of the operating principles of cyclone separator 10 in terms of its application to removing entrained particles from an air stream in a vacuum cleaner.
Cyclone separator 10 has an inlet pipe 12 and a main body comprising upper cylindrical portion 14 and lower frusto-conical portion 16. The particle laden air stream is injected through inlet pipe 12 which is positioned tangentially to upper cylindrical portion 14. The shape of upper cylindrical portion 14 and frusto-conical portion 16 induces the air stream to spin creating a vortex. Larger or more dense particles are forced outwards to the walls of cyclone separator 10 where the drag of the spinning air as well as the force of gravity causes them to fall down the walls into an outlet or collector 18. The lighter or less dense particles, as well as the air medium itself, reverses course at approximately collector G and pass outwardly through the low pressure centre of separator 10 and exit separator 10 via air outlet 20 which is positioned in the upper portion of upper cylindrical portion 14.
The separation process in cyclones generally requires a steady flow free of fluctuations o. short term variations in the flow rate. The inlet and outlets of cyclone separators are typically operated open to the atmosphere so that there is no pressure difference between the two. If one of the outlets must be operated at a back pressure, both outlets would typically be kept at the same pressure.
When a cyclone separator is designed, the principal factors which are typically considered are the efficiency of the cyclone separator in removing particles of different diameters and the pressure drop associated with the cyclone operation. The principle geometric factors which are used in designing a cyclone separator are the inlet height (A); the inlet width (B); the air outlet diameter (C); the outlet duct length (D); the cone height (Lc); the dirt outlet diameter (G); and, the cylinder height (L)
The value d.sub.50 represents the smallest diameter particle of which 50 percent is removed by the cyclone. Current cyclones have a limitation that the geometry controls the particle removal efficiency for a given particle diameter. The dimensions which may be varied to alter the d.sub.50 value are features (A)-(D), (G), (L) and (Lc) which are listed above.
Typically, there are four ways to increase the small particle removal efficiency of a cyclone. These are (1) reducing the cyclone diameter; (2) reducing the outlet diameter; (3) reducing the cone angle; and (4) increasing the body length. If it is acceptable to increase the pressure drop, then an increase in the pressure drop will (1) increase the particle capture efficiency; (2) increase the capacity and (3) decrease the underflow to throughput ratio.
In terms of importance, it appears that the most important parameter is the cyclone diameter. A smaller cyclone diameter implies a smaller d.sub.50 value by virtue of the higher cyclone speeds and the higher centrifugal forces which may be achieved. For two cyclones of the same diameter, the next most important design parameter appears to be L/d, namely the length of the cylindrical section 14 divided by the diameter of the cyclone and Lc/d, the length of the conical section 16 divided by the width of the cone. Varying L/d and Lc/d will affect the d.sub.50 performance of the separation process in the cyclone.
Due to its intended use, a vacuum cleaners is designed to filter particles of varying sizes from an air stream. With most vacuum cleaners on the market, a filter material such as a paper bag is used to filter the air. The bag will remove from the air stream any particle larger than the size of the pore in the bag. Thus only a single stage of filtration may be employed. However, if a cyclone is used in a vacuum cleaner, then multiple filtration stages may be employed. This is due to the fact that particle sizes which are generally to be filtered by a vacuum cleaner take on a spectrum of values that necessitates that a plurality of cyclonic separators be used in a series. For example, the first cyclonic separator in a series may have a large d.sub.50 specification followed by one with a smaller d.sub.50 specification.
For example, in U.S. Pat. No. 3,425,192, a vacuum cleaning assembly was disclosed which used a first frusto-conical cyclone and six secondary cyclones.
More recently, cyclonic technology has been improved and introduced commercially into canister and upright vacuum cleaners. See for example U.S. Pat. Nos. 4,373,228; 4,571,772; 4,573,236; 4,593,429; 4,643,748; 4,826,515; 4,853,008; 4,853,011; 5,062,870; 5,078,761; 5,090,976; 5,145,499; 5,160,356; 5,255,411; 5,358,290; 5,558,697; and RE 32,257. These patents disclose a novel approach to vacuum cleaner design in which sequential cyclones are utilized as the filtration medium for a vacuum cleaner. Pursuant to the teaching of these patents, the first sequential cyclone is designed to be of a lower efficiency to remove only the larger particles which are entrained in an air stream. The smaller particles remain entrained in the air stream and are transported to the second sequential cyclone which is frusto-conical in shape. The second sequential cyclone is designed to remove the smaller particles which are entrained in the air stream. If larger particles are carried over into the second cyclone separator, then they will typically not be removed by the cyclone separator but exit the frusto-conical cyclone with the air stream.
One disadvantage of cyclonic vacuum cleaners is the amount of power which is required to create an air flow sufficient to convey the dirty air through the cyclones at sufficient speeds to maintain the air flowing cyclonically through the cyclones.